1. Field of the Invention
The invention relates to a model and method for designing the output stage of an electronic ballast using a computer.
2. Description of the Related Art
As shown in FIG. 1, present day electronic ballasts include circuitry for filtering electromagnetic interference (EMI) to block ballast generated noise, power factor correction (PFC) circuitry for sinusoidal input current, undervoltage lockout (UVLO) and fault protection circuitry, a half-bridge switch with driver and timing circuitry for high-frequency operation, and a final output stage to power the lamp.
FIG. 2 shows a simplified model of the output stage of a typical fluorescent lamp circuit. The lamp requires a current for a specified time to preheat the filaments, a high-voltage for ignition, and running power. These requirements are satisfied by changing the frequency of the input voltage and properly selecting V.sub.in, L and C. For preheat and ignition, the lamp is not conducting and the circuit is a series L-C. During running, the lamp is conducting, and the circuit is an L in series with a parallel R-C.
The magnitude of the transfer function (lamp voltage divided by input voltage) for the two RCL circuit configurations, shown in FIG. 3, illustrates the operating characteristics for this design approach. The currents and voltages corresponding to the resulting operating frequencies determine the maximum current and voltage ratings for the inductor, capacitor and the switches which, in turn, directly determine the size and cost of the ballast.
It would be desirable to provide a computer program for automatically designing the output stage and specifying the values of various components of an electronic ballast, such as the inductor and capacitor of the output stage, based on certain parameters specified by the user.